It has already been known that a double layer membrane comprising an anion exchange layer and a cation exchange layer, functions as a so-called bipolar membrane in such a manner that when it is disposed so that its anion exchange layer side faces an anode, and an electric current is applied while a solution containing a neutral salt or alkali is contacted to the anode sided and a neutral salt or acid solution is contacted to the cathode side, water will decompose and dissociate into a hydroxyl ion and a hydrogen ion at the interface of the double layer membrane, whereupon hydroxyl ions will be formed on the anode side and hydrogen ions will be formed on the cathode side.
Further, a method is reported wherein using a unit comprising an anion exchange membrane, a bipolar membrane and a cation exchange membrane, a neutral salt is subjected to double decomposition by electrodialysis to produce an acid and an alkali. Many proposals have been made also for the preparation of a bipolar membrane useful as a water splitter which constitutes the essential element for the above-mentioned double decomposition method of a neutral salt.
For example, Japanese Examined Patent Publication No. 31860/1985 and Japanese Unexamined Patent Publications No. 205135/1987, No. 205135/1987 and No. 95235/1988 disclose a method for introducing cation exchange groups by a treatment such as sulfonation on one side of a benzene ring-containing hydrocarbon-type polymer membrane and introducing anion exchange groups such as quaternary ammonium groups on the other side.
Further, Japanese Examined Patent Publication No. 35936/1985 discloses a membrane obtained by casting a polyvinylidene fluoride solution having fine particulate cation exchange resin dispersed therein, on a cast film of a quaternary ammonium salt formed from a mixed solution of polyvinylbenzyl chloride and polyvinylidene fluoride. Japanese Examined Patent Publication No. 47235/1984 discloses a membrane having an inorganic compound such as iron chloride interposed at the laminated interface between an anion exchange membrane and a cation exchange membrane.
Further, Japanese Unexamined Patent Publication No. 1234/1985 discloses a non-crosslinked fluorine-containing bipolar membrane, wherein pendant chains of one of the layers have cation exchange groups and pendant chains of the other layer have anion exchange groups, and the backbone chain is made of a perfluorocarbon polymer.
On the other hand, it is known, for example, from Japanese Examined Patent Publication No. 28849/1985 and Japanese Unexamined Patent Publication No. 86821/1980 that these bipolar membranes are useful as ion-selective separating membranes for selective separation among ions of the same electrical charge. However, nothing has been known with respect to the selective permeability of hydrogen ions.
Further, as a membrane for selective permeation of monovalent cations, especially for selective permeation of sodium ions from sea water, it has been proposed to adsorb or bond a compound having an opposite electrical charge on the surface of a cation exchange membrane, for example, in Japanese Examined Patent Publications No. 23607/1971, No. 42082/1971, No. 3801/1972 and No. 38310/1972. Japanese Examined Patent Publication No. 3081/1972 and Japanese Unexamined Patent Publication No. 205135/1987 further disclose crosslinking or polymerization treatment after bonding the compound having the opposite electrical charge. Further, Japanese Examined Patent Publication No. 27084/1986 discloses a membrane having a thin layer of a nitrogen-containing compound formed on the surface of a cation exchange membrane by plasma polymerization. However, nothing has been indicated with respect to the hydrogen ion-selective permeability of these monovalent cation-selective permeable membranes.
With conventional monovalent ion-selective separating membranes for selective permeation of sodium ions from sea water, the permeability of sodium ions is high, and the selective permeability of hydrogen ions is inadequate. On the other hand, with conventional bipolar membranes, water is likely to be decomposed, thus leading to a drawback that hydroxyl ions formed by the decomposition of water will react with polyvalent cations contained in the acid solution as impurities to form hardly soluble precipitates.
There has been a substantial demand for a method for electrically dialyzing hydrogen ions selectively for e.g. electrodialysis for e.g. recovery of a concentrated acid from a waste acid solution or removal of an acid. Nevertheless, there has been no practical method which fully satisfies such a demand.